The present invention relates to a single screw extruder comprising a barrier screw and a barrel in which the barrier screw is rotatably mounted and which comprises at least a feed zone longitudinal section and a melting zone longitudinal section. The invention further relates to a method for extruding plastic material using such a single screw extruder.
A single screw extruder having a barrier screw is for example disclosed in the paper xe2x80x9cZylinder glatt oder genutetxe2x80x9d, Maschinenmarkt, Wxc3xcrzburg 95 (1989) 48, pages 40-43. In this paper different concepts of barrier screws are presented which are used in extruders having smooth or grooved feed areas. The barrels in the area of the melting zone longitudinal sections are formed in a plane and smooth manner for a better separation of melt and solid matter.
A screw housing for an extruder is known from DE 25 14 307, which comprises at least one groove extending across the whole length. This groove serves to avoid a xe2x80x9cscrew slipxe2x80x9d, that is a rotation of the screw without a transport of material in the barrel. The screw shown in this document is not a barrier screw.
Generally single screw extruders are known. They typically comprise an extruder plastification barrel in which a screw is rotatably mounted. A powdered or granulated initial material (with stabilizers, slip additives, if required thermoplastic materials being added with fillers and colorings) is fed at one end of the barrel and is transported by the rotating screw through the barrel to a so-called demoulding tool or matrix located at the opposite end. At first, the initial material is conveyed or transported through the so-called feed zone and is thereby compressed. This feed zone is followed by a melting zone and plastification zone, respectively, in which the initial material is melted by friction at the barrel inner surface. The melting process mayxe2x80x94particularly when starting the extruderxe2x80x94be supported by an outer heating of the barrel via (mostly electrical) heating elements. Depending on the application the melting zone is followed by a homogenizing zone and a demoulding zone in which the plastified material is prepared for further processing.
In recent years, single screw extruders being based on the so-called barrier screw concept have gained acceptance to an increasing extent. In a so-called barrier screw, the screw channel is divided in a solid matter channel and a melt channel by way of a further side bar. Compared with the primary side bar, being known from conventional screws, the smaller formed barrier side bar allows a transverse flow of melt from the solid matter channel into the melt channel. The cross-section of the melt channel increases in a downstream direction whereas the cross-section of the solid matter channel continuously decreases in the downstream direction as to maintain the desired conveying effect of the screw. Due to the so-called barrier zone particularly the heat transfer from the barrel and the screw surface to the granulate not yet melted is improved.
Although this barrier screw concept has been proven practical for many applications still remains it the need to increase the performance of the single screw extruder with barrier screw while keeping the necessary structural measures as low as possible.
The object of the present invention is, therefore, to improve the single screw extruder based on the barrier screw concept so that the performance, particularly the output and the pressure build-up capability as well as the melting performance, is increased.
The object underlying the present invention is solved by a single screw extruder of the aforementioned kind in that the barrel comprises at least one groove extending in a longitudinal direction and formed in the barrel""s inner surface in the area of the melting zone longitudinal section.
By way of combining a barrier screw with a barrel comprising at least one groove along the melting zone, a substantial increase of output compared to conventional barrier screw extruders is achievable. This is particularly the result of an improvement of the melting process of the initial material. This may substantially be put down to the fact that the solid matter gets caught in the groove and, therefore, enters the melt channel only in small quantities. This also leads to the advantage that the heat transfer is substantially improved since the barrel surface in the area of the grooves comes in contact with the solid matter. In the prior known barrels with a smooth surface the melt quasi forms a heat barrier, at least said smooth surface made worse the heat transfer from the barrel surface to the inside.
The mentioned improvement of the melting process and the heat transfer allows to increase the rotational speed directed output without having to provide additional structural measures. In view of the prior single screw extruders it was necessary for increasing the output to increase the length of the melting zone and/or the rotational speed of the screw. However, an increased rotational speed results in an undesired heating process of the melt in the extruder which may cause damages of the initial material. Otherwise the melt must be more cooled down afterwards which in turn results in additional structural measures.
Additional structural measures are therefore not necessary with the single screw extruder according to the present invention. The object of the present invention is hence completely solved.
In addition to the aforementioned advantages the single screw extruder with barrier screw according to the present invention has the advantage that the pressure build-up capability is improved. Compared to the known single screw extruders it is not necessary any more to build up a very high pressure in the so-called feed zone as to enable a predetermined lower pressure at the end of the single screw extruder. With the single screw extruder according to the present invention it is possible to substantially reduce the pressure between the feed zone and the melting zone. This in turn results in a reduction of wear of the screw in the transition area between the feed zone and the melting zone since it is operated with lower pressures.
Due to the pressure reduction in the area of the feed zone it does not have to be constructed any more as complicated as in prior solutions and does not have to be provided with cooling means and a xe2x80x9cheat separationxe2x80x9d towards the heated melting zone. Advantageously, the feed zone and the melting zone may therefore be formed individually.
In a preferred embodiment of the present invention the groove extends parallel to the longitudinal axis of the barrel (axial groove), wherein more preferably the grooves extend helically in the feed zone.
It has been shown that the axially extending form of the groove provides optimal results particularly in view of the output and the pressure build-up, however, without deteriorating the melting process and the homogeneity of the melt, respectively. Of course, a helically arranged groove may also be contemplated.
In a preferred embodiment the barrel comprises several grooves being equally spaced apart in circumferential direction which grooves extend preferably parallel to the longitudinal axis of the barrel.
This has the advantage that the process of matching the grooves is simplified compared with the groove helically arranged.
In a further preferred embodiment the width and/or the depth of the groove varies in a longitudinal direction, preferably the grooves depth decreases towards the downstream end of the melting zone section, preferably to zero.
In a further preferred embodiment of the invention at least one groove is provided also in the barrel inner surface (innerwall) in the area of the feed zone section, the groove extending parallel or helically relative to the longitudinal axis. Preferably, the groove in the area of the feed zone section leads into the groove in the area of the melting zone without transition. Preferably both grooves have the same lead angle.
This has the advantage that the groove extends continuously without any break along the feed zone and melting zone thereby further improving the output and the pressure build-up.
In a preferred embodiment the barrel is formed as a one-piece barrel and is preferably provided with a constant inner diameter. This measure has the advantage that the conventional and structurally complicated separation into a feed zone with a cooled grooved liner and a heated melting zone may be omitted. Thereby manufacturing costs may be saved. Moreover, due to the good output of the overall system a cooling of the feed zone may be omitted which leads to operation cost savings.
In a further preferred embodiment the barrier screw is formed with two or more channel pairs so that two or more solid matter channels and two or more melt channels are created.
This has the advantage that the melting performance of the single screw extruder may be improved compared with the one channel paired barrier screw. Additionally, the abrasive wear at the primary side bar of the screw may be minimized thereby.
The object underlying the present invention is also solved by a method of the aforementioned kind which is characterized in that in the area of the melting zone solid plastic material (solid matter) is transported in a defined quantity out of the solid matter channel into the melt channel. Preferably, the solid matter is transported out of the solid matter channel into the melt channel at predetermined locations along the barrel. Most preferred, the transport of the predetermined quantity of solid matter is caused by the pressure difference between the solid matter channel and the melt channel.
Further advantages and embodiments of the invention will be apparent from the following description and the drawings.
It is understood that the features recited above and those yet to be explained below can be used not only in the respective combination indicated, but also in other combinations or in isolation without leaving the context of the present invention.